Certain wind turbines are known to comprise a tower, a nacelle, a hub, and a plurality of blades. The nacelle typically houses an electric machine, and devices for controlling the wind turbine.
The nacelle is fitted to the tower to rotate about a substantially vertical axis, and supports the hub for rotation.
Each blade comprises a reinforcing beam, extending substantially the whole length of the blade, and a shell defining an airfoil. The shell is fixed to the reinforcing beam, and is usually made of composite material with a resin matrix reinforced with glass or carbon fibers.
The blades in turn are fitted to the hub in rotary manner, to enable adjusting the attack (or pitch) angle.
More specifically, the blades are connected to the hub by a blade roots, which substantially serves to transmit loads between the blade and the hub. Moreover, the connection of blade root to the hub by a bearing enables adjusting the orientation and pitch angle of the blade.
The blade root is normally an annular element, and may either be integral with the body of the blade or produced separately and joined to the blade body. The root and the blade body are made from the same composite material. The blade root, however, comprises longitudinal reinforcing elements to withstand the severe loads caused by the weight (several tons) and length (tens of meters) of the blade and prevent structural damage or severe deformation. More specifically, the blade root has an outer annular shell and inner annular shell, both made of composite material, and between which the longitudinal reinforcing elements are incorporated.
The blade root also comprises fasteners for connection to an outer metal ring, which may be a bearing race or a structural element in turn connected to the hub by a bearing.
According to one known solution, described in European Patent No EP 1633624 B1, threaded bushings tapering towards their distal end (with respect to the hub (i.e., towards the blade tip)) serve both as longitudinal reinforcing elements and as elements for fastening to an outer metal ring for connection to the hub. The bushings are fixed individually between the inner and outer annular shells, and are connected to a bearing on the hub by respective studs.
Known solutions are especially critical as regards transmitting severe loads between the blade body, the reinforcing elements, and the hub. That is, stress is transmitted to the blade via the longitudinal reinforcing elements, which only interact with one another via the annular shell material. This may therefore result in severely unbalanced load distribution, especially at certain angles during rotation. In the event of even microscopic defects, and at times even with a substantially sound structure, forces may concentrate to such an extent as to produce or propagate damage in the composite material.